Hi Jamie,
I have the same Zoom ADSL X3 modem, with exactly the same firmware revision. I've also signed up to Plus Net, and have had exactly the same problems as you.
The only difference is, that I have on occasion been able to stop the constant disconnects. I have a linux gateway server, and seemed to start having problems after turning on the half-bridge.
At one point this evening, it was disconnecting every 2 minutes (almost exactly). I've tried the connection with other hardware, and it's fine, so that would lead me to believe that the the Zoom is at fault.
So far, my connection has been back up for around 45 min, so here's hoping!
Hope you get your problem solved... think I'll be swapping for a Linksys.
Cheers,
Rich.